The present invention relates to signal transfer methods to support parallel processing in a large number of integrated circuits, and particularly to methods to support wafer level testing or wafer level calculations of integrated circuits.
Current art integrated circuit (IC) fabrication techniques involve formation of a plurality of individual IC devices on a single semiconductor substrate, termed a xe2x80x9cwaferxe2x80x9d. After fabrication is completed, the wafer is scribed to separate the individual IC devices called xe2x80x9cdicexe2x80x9d. Usually the individual dice are spaced apart from one another on the wafer to accommodate the scribing tool used to cut the wafer. The wafer thus has the appearance of a series of IC dice separated by intersecting lines to accommodate the scribing operation. These lines are commonly referred to as xe2x80x9cscribing lanesxe2x80x9d. For cost saving purpose, it is desirable to test the dice while they are still in wafer form (called xe2x80x9cwafer level testingxe2x80x9d). The major difficulty for wafer level testing is the need to establish connections between the tester and the input or output (I/O) signals in each die. Typically, wafer level testing is performed by placing a series of probe needles in contact with bonding pads that are formed on an exposed metal surface of each IC die. These bonding pads are also used to connect elements of a lead frame if the IC die is subsequently packaged. An expensive stepping device moves the probe needles to connect different dice for a tester to test them one by one. Defective dice are marked with ink after they failed such wafer level tests. Unfortunately, individual dice that have passed wafer level tests may still fail in later continuous operation due to reliability problems. A common practice in the IC industry to detect reliability problems is called xe2x80x9cburn-inxe2x80x9d. During burn-in tests, IC devices are exercised at elevated temperature and elevated power supply voltage. It is known that IC dice pass these bum-in tests are highly reliable in practical operation conditions. Conventional burn-in tests are usually done after the IC dice are packaged because of the difficulty in using probe stepping devices in those harsh burn-in conditions.
It is desirable to avoid using a costly stepping probe tester for wafer level tests. It is even more desirable to do burn-in tests at wafer level. The major obstacle for wafer level testing is the difficulty to transfer data between the tester and the individual dice on a wafer. One method is to use a probing device that provides all necessary connections to all the dice on a wafer. Such probing device would have thousands of probe needles and metal lines. It is not practical to build such complex probing devices. Another approach is to transfer testing data into and out of each die through conductive lines patterned on the wafer. This approach is also very difficult. The insulator materials used to separate conductor layers in IC (called interlayer dielectric) have a strong tendency to absorb water moisture, which is known to cause reliability problems. It is a common practice to cover the wafer with a layer of water-resist thin film. This water-resist layer can be destroyed during wafer scribing so that moisture still can penetrate through the exposed edges of scribed dice. A common solution to this problem is to build a continuous metal wall (called xe2x80x9cseal ringxe2x80x9d) between internal circuits and scribing lanes. Combination of the seal ring and the water-resist layer provides a complete water-resist shield for scribed dice. In the mean time, the seal ring also becomes a barrier for all conducting layers used in normal IC fabrication procedures. It is therefore necessary to use additional procedures to deposit wafer level connection lines after all normal IC fabrication procedures have been done. One example of such approach was proposed in U.S. Pat. No. 5,053,900 to W. Parrish. This patent describes the formation of multiple conductive lines along the scribing lanes of a wafer after normal IC fabrication processes are done. These conductive lines connect enlarged I/O pads at the edges of the wafer with suitable multiplexing circuitry formed in an otherwise unused circuit of the wafer. The conductive lines connect the I/O pads of the individual IC dice to the multiplexing circuitry. Wafer level testing is then performed by placing a single set of test probes in contact with a set of enlarged I/O pads associated with the multiplexing circuitry. The multiplexing circuitry selectively connects the test probes with the individual IC dice to be tested through the wafer level conductive lines. These conductive lines would be destroyed by the subsequent die scribing processes. Because there are a large amount of metal in the scribing lane, some of the I/O pads of the individual IC dice may be electrically shorted after the scribing process. Slivers of conductive materials may remain in proximity to sensitive regions of the IC dice. These slivers may interfere with subsequent bonding operations by shorting an IC die with unintended conductive bridges between adjacent I/O pads on the die. In U.S. Pat. No. 5,532,174, Corrigan describes a method to solve the problems caused by scribed metal lines. Corrigan provides the wafer level conductive lines using a sacrificial conductive layer that is removed from the wafer by etching before the scribing process. To facilitate its removal, this conductive layer is formed from a conductive material differing from the conductive material employed to form the I/O pads of the IC dice. Another approach is described in U.S. Pat. No. 5,399,505 to Dasse et al. Wafer level connections are formed after normal IC fabrication procedures to connect probe points to the bonding pads of a plurality of IC dice. External probe needles connected to those probe points provide testing connections to test a plurality of dice, while the bonding pads in each die remain ready for subsequent bonding processes. In U.S. Pat. No. 5,593,903 Beckenbaugh et al. describe methods to deposit multiple layers of metals and insulators on semiconductor wafers after normal IC fabrications are done. The wafer conductors are electrically coupled to bonding pads on each of a plurality of IC die on the wafer at a first end and to wafer test pads at the periphery of the wafer at the second end. Thus, the wafer conductors, wafer test pads and contact pads allow each integrated circuit die to be accessed individually for electrical testing. When all the testing conductors are removed after testing, the bonding pads of each IC die are returned to the same condition they had prior to the formation of the testing conductors. All of the above inventions require additional manufacture procedures to build wafer level connections. These additional procedures increase manufacture cost. They also introduce additional yield loss. These wafer level conductive lines need to connect the bonding pads in all IC dice on a wafer. The most popular wafer size for the current art IC technologies is 8 inches, and the industry is moving into 12-inch wafer. There are thousands of dice in each current art wafer. The wafer level connections will need to use thousands of 8-inch or 12-inch long lines to connect all dice on each wafer. These conductive lines occupy a large area on the wafer. It is therefore likely to cause additional yield loss at subsequent scribing process. The etching processes to remove testing conductor lines are equally likely to cause additional yield loss. Due to the resistance-capacitance propagation delays (RC delays) of those large area testing lines, it is very difficult to do high frequency tests using such large area conductive lines. All of those inventions provide testing methods to test one die at a time. Those inventions provide little improvement in testing time while testing time is usually the dominating factor that defines testing cost. All the above methods are useful only for wafer level tests or burn-in tests; they are not supporting the actual applications of the IC products.
It is therefore highly desirable to provide wafer level data transfer methods using a small number of small area conductive lines. It is also desirable to support parallel testing so that a large number of dice can be tested simultaneously. Testing time, and therefore testing cost, can be reduced significantly. The wafer level data transfer methods are not only useful for testing purpose. It is even more desirable to provide extremely powerful parallel processing IC products using wafer level connections.
The primary objective of the present invention is to provide an effective data transfer method to support parallel operations in a large number of IC dice. One objective of this invention is to simplify the connections to support wafer level tests. The other objective is to test a large number of dice in parallel to reduce testing cost. Another important objective of the present invention is to provide the flexibility to avoid defective circuits. Yet another objective is to provide wafer level connections without using additional fabrication processes. The other primary objective of this invention is to build multiple dice integrated circuits to achieve unprecedented performance. These and other objectives of the present invention are achieved by inter-dice data transfer methods of the present invention. Each individual die of the present invention contains internal circuits to control data transfer to nearby dice. Wafer level data transfer is achieved by a series of inter-dice data transfers. The distance between the drivers and the receivers of inter-dice data transfer circuits of the present invention is very short. It is therefore possible to use a small number of small area wafer level conductive lines to support wafer level parallel processing activities. The metal lines in the scribing lane can be short and narrow. They are unlikely to cause electrical shorts during scribing process. External connections are provides by short conductive lines at the peripherals of a wafer. It is often possible to use a small number of external signals to control parallel processing for thousands of dice. The control logic in each die also can be programmed to avoid defective circuits in the wafer. It is therefore possible to build an IC containing many dice with excellent yields.